Continuing the past decade's investigation of the role of serum binding proteins, especially thyroxine binding alpha-globulin (TBG), newer techniques such as affinity chromatography will be added to the standard protein fractionation procedures already successfully utilized. In addition to ongoing studies of the circulating transport protein, TBG, the research effort has been broadened to include intracellular binding proteins in the investigation of the molecular mechanism of thyroid hormone action at the tissue level. The investigation will compare tissues in the rat, and eventually some human tissues with special emphasis on comparison of "responsive" and "unresponsive" tissues. Since it has been shown that the brain, spleen, and testes of the experimental rat do not manifest increased oxygen consumption after administration of thyroid hormones in vivo, the binding proteins of the nucleus, mitochondria and cytosol of these tissues will be studied in comparison with representative typically responsive tissues including liver, kidney, myocardium and striated muscle. The preliminary findings of direct thyroid hormone action upon the mitochondria will be more fully explored, including especially the further characterization of the specific high affinity, low capacity receptor protein discovered in the mitochondrial membrane. Investigation will be made of the direct action of thyroid hormone upon isolated mitochondria in vitro with respect to increased oxygen consumption, adenosine triphosphate (ATP) formation and the mechanism of these effects. Concentrations of thyroxine (T4) and triiodothyronine (T3) considered to be physiologically significant (i.e., nanomolar or picomolar levels) will be employed.